Simple to keep Luke on Tattooine. If they had told him his fathers true origin he would have wanted to go there. The way it was Owen had Luke staying with the farm. Obiwan had Luke staying by saying his father was there. Luke most likely didn't have any want to go to Alderaan. Thus he was out of danger from Vader(being that Vader and, Anakin were both Obiwan's apprentice's. They would both have knowledge of Obiwan and, Owen being on Alderaan.). Also the thing that might make Owen not wanting Luke end up as his father has could be that he was killed by Vader and, he doesn't want Luke to be killed by Vader. Then of course the message from Leia came and, it was time to come out of hiding. Though, if you really want to have Obiwan saying the complete truth in the hut. He could have just been repeating one of Owen's statements word for word:
"He should have stayed here and, not gotten involved" Owen say's to Obiwan after hearing the news of Anakin's death by Vader while in his Alderaan home.

Though, I do admit my idea doesn't leave room for the Clone wars. I guess becuase, I didn't think about that until the prequels came along..... Ah well *shrug*.

Though, what do you mean by "Owen is obviously a long-time resident of Tatooine "? Aside from the prequels and, deleted scenes there isn't much to indicate he's been there longer then Luke has.